The present invention relates generally to magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) and, more particularly, to coils for use in an MRI host device.
Local coils, surface coils and the like, are currently made in two forms: xe2x80x9creceive only,xe2x80x9d and xe2x80x9ctransmit/receive.xe2x80x9d Local coils that are receive only generally use the MRI system body coil for excitation. This has the benefit of generally very uniform RF excitation, and therefore uniform tip angle and image uniformity, but the disadvantage of increased RF power deposition in the body of the patient, and potential problems with aliasing of undesired signal from outside of the imaging volume. For a transmit/receive local coil, the local coil itself provides the RF excitation to the region of interest. Transmit/receive local coils therefore improve the situation from the perspective of aliasing and power deposition, but generally produce a less uniform RF excitation field, and therefore less uniform distribution of tip angle over the imaging volume, and subsequently an image with reduced uniformity. The following discussion applies to both the single element, single receiver coils, and multiple element, multiple receiver coils, including phased array coils.
The practitioner""s decision between techniques providing better uniformity and those providing reduced aliasing effects and power deposition is dependent upon the imaging technique employed, the target anatomy, and the aspects of the resulting image deemed most important for the particular scan. It would therefore be advantageous to be able to use a local coil as a receive only coil when the imaging process is best served by this, and as a transmit/receive coil when that configuration provides the best benefit.
In accordance with a first aspect of the present invention a local or surface coil is provided that can be user selected to be a receive only coil, or a transmit/receive coil, as desired. In its various embodiments, the invention may provide a basic coil with a manual mode [receive only or transmit/receive] selector or an electronic mode selector. Alternatively, the local coil may provide automatic electronic mode selection from an operation point command entered as a part of a scan protocol.
The invention consists of a local or surface coil that includes decoupling circuitry to allow it to operate as a receive only coil in a host MRI system, the system body coil typically being the transmit signal source. The local or surface coil also includes circuitry to defeat the decoupling circuitry for both low power [receive] and high power [transmit] functions to allow the same physical coil to be used as a transmit/receive coil. The two modes of the coil thus allow the user to select the mode of local or surface coil operation best suited to the particular type of imaging being performed.
Selection may be by manual means requiring operator intervention, or, alternatively, by automatic means via sensing of the coil type desired by the system operator from the bias pattern of the host MRI system, or other means. The selection means places the decoupling networks on the coil in the non-decoupled mode continuously for transmit/receive operation; for receive only operation, the decoupling networks are in the non-decoupled mode only for receive, and in the decoupled state during the application of the external transmit RF field. The decoupling networks are designed to operate in the non-decoupled mode under the electrical strain of the applied transmit power when the coil is used as a transmit/receive device, and also to withstand the effects of the external transmit RF field when in the decoupled mode [the coil operating as a receive only coil at this time].